user32.dll SendMessage VolumeUp and VolumeDown issue

Question

I'm trying to make a Unity3D Application that can control the volume of another application. So far I've gotten it to work to change the volume of ALL applications, but I'm having trouble to get it to change just one, e.g. Spotify. This is on Windows 8, but I'd like it to work for Windows 7 as well.

private const int APPCOMMAND_VOLUME_UP = 0xA0000;
private const int APPCOMMAND_VOLUME_DOWN = 0x90000;
private const int WM_APPCOMMAND = 0x319;

[DllImport("user32.dll")]
public static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

[DllImport("user32.dll")]
public static extern IntPtr FindWindow(string strClassName, string strWindowName);

[DllImport("user32.dll", SetLastError = true)]
public static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint processId);

[DllImport("user32.dll")]
public static extern IntPtr GetActiveWindow();

public void DecreaseVolume()
{
    IntPtr hWnd = FindWindow("SpotifyMainWindow", "Spotify");

    if (hWnd == IntPtr.Zero)
        return;

    uint pID;
    GetWindowThreadProcessId(hWnd, out pID);

    Debug.Log(pID);

    SendMessage(hWnd, WM_APPCOMMAND, hWnd, (IntPtr)APPCOMMAND_VOLUME_DOWN);
}

public void IncreaseVolume()
{
    // same as above, but with APPCOMMAND_VOLUME_UP
}

I'm pretty sure I'm getting the correct handle (hWnd) from FindWindow() because the pID printed out matches the PID of Spotify in my Task Manager, however all this does is change the volume of everything instead of just Spotify. This is the same result as if I didn't specify a specific handle e.g.

SendMessage(IntPtr.Zero, WM_APPCOMMAND, IntPtr.Zero, (IntPtr)APPCOMMAND_VOLUME_DOWN);

or if I use GetActiveWindow().

I've never messed around with these windows api stuff until today, so any help would be very appreciated!